1. Field of the Invention
This invention relates to an image-forming apparatus such as a display apparatus using electron beams and, more particularly, it relates to an image-forming apparatus comprising spacers arranged within the envelope of the apparatus to internally support the envelope against the atmospheric pressure.
2. Related Background Art
There have been known two types of electron-emitting devices, which are the thermionic electron source and the cold cathode electron source. Cold cathode electron sources refer to the field emission type (hereinafter referred to as the FE type), the metal/insulation layer/metal type (hereinafter referred to as the MIM type) and the surface conduction electron-emitting type (hereinafter referred to as the SCE type).
Examples of SCE type device include the one proposed in M. I. Elinson, Radio Eng. Electron Pys., 10 (1965).
An SCE type device is realized by utilizing the phenomenon that electrons are emitted out of a thin film with a small area formed on a substrate when an electric current is forced to flow in parallel with the film surface. While Elinson proposes the use of SnO.sub.2 thin film for a device of this type, the use of Au thin film is proposed in G. Dittmer: "Thin Solid Films", 9, 317 (1972) whereas the use of In.sub.2 O.sub.3 /SnO.sub.2 thin film and that of carbon thin film are also discussed respectively in M. Hartwell and C. G. Fonstad: "IEEE Trans. ED Conf.", 519 (1975) and H. Araki et al.: "Vacuum", Vol. 26, No. 1, p. 22 (1983).
FIG. 30 of the accompanying drawings schematically illustrates a typical surface conduction electron-emitting device proposed by M. Hartwell. In FIG. 30, reference numeral 3001 denotes an insulating substrate. Reference numeral 3004 denotes an electron-emitting region-forming thin film, which is a thin metal oxide film prepared by sputtering, using an H-shaped pattern, in which an electron-emitting region 3005 is produced when it is subjected to an electrically energizing process referred to as "energization forming" as will be described hereinafter. In FIG. 30, a pair of device electrodes are separated by a length L of 0.5 to 1[mm] and have a width W of 1.0[mm].
Conventionally, an electron emitting region 3005 is produced in a surface conduction electron-emitting device by subjecting the electron-emitting region-forming thin film 3004 of the device to an electrically energizing process, which is referred to as energization forming. In the energization forming process, a constant DC voltage or a slowly rising DC voltage that rises typically at a very slow rate of 1 V/min. is applied to given opposite ends of the electron-emitting region-forming thin film 3004 to partly destroy, deform or transform the film and produce an electron-emitting region 3005 which is electrically highly resistive. Thus, the electron-emitting region 3005 is part of the electron-emitting region-forming thin film 3004 that typically contains a fissure or fissures therein so that electrons may be emitted from the fissure and its vicinity. The electron-emitting region-forming thin film 3004 including the electron-emitting region produced by energization forming will be referred to as the electron-emitting region-containing thin film. Note that, once subjected to an energization forming process, a surface conduction electron-emitting device comes to emit electrons from its electron emitting region 3005 whenever an appropriate voltage is applied to the electron-emitting region-containing thin film 3004 to make an electric current run through the device.
Examples of FE type device include those proposed by W. P. Dyke & W. W. Dolan, "Field emission", Advance in Electron Physics, 8, 89 (1956) and C. A. Spindt, "PHYSICAL Properties of thin-film field emission cathodes with molybdenum cones", J. Appl. Phys., 47, 5284 (1976). FIG. 31 of the accompanying drawings shows a schematic cross sectional view of a device proposed by C. A. Spindt et al., which is a typical FE type device. Referring to FIG. 31, it comprises a substrate 3010, an emitter wiring layer 3011 made of a conductive material, a conical emitter 3012, an insulation layer 3013 and a gate electrode 3014. The device emits electrons from the tip of the conical emitter 3012 as an appropriate voltage is applied to the conical emitter 3012 and the gate electrode 3014.
While the FE type device illustrated and described above has a multilayer structure, the emitter and the gate electrode may alternatively be arranged on the substrate substantially in parallel with the plane of the substrate.
Examples of MIM type device are disclosed in papers including C. A. Mead, "The tunnel-emission amplifier", J. Appl. Phys., 32, 646 (1961). FIG. 32 of the accompanying drawings shows a schematic cross sectional view of a typical MIM type device. Referring to FIG. 32, it comprises a substrate 3020, a lower metal electrode 3021, a thin insulation layer 3022 having a thickness of about 10[nm] and an upper metal electrode having a thickness of about 30[nm].
The MIM type device emits electrons from the surface of the upper electrode 3023 as an appropriate voltage is applied between the upper electrode 3023 and the lower electrode 3021.
Contrary to a thermionic device, a cold cathode device is adapted to emit electrons at low temperature and hence does not need a heater. Consequently, the former has a simplified structure if compared with the latter and, therefore, it is possible to prepare very small cold cathode devices, which are relatively free from problems such as a thermally molten substrate if they are densely arranged on a substrate.
Additionally, while the responsiveness of a thermionic device is defined by that of the heater used for it, a cold cathode device is free from such a problem and hence a highly responsive cold cathode device can be realized without difficulty.
In view of the above listed advantages and other advantages, efforts have been paid to develop electron beam apparatus, image-forming apparatus in particularly, comprising cold cathode devices.
Particularly, the surface conduction electron-emitting device provides a remarkable advantage that a large number of devices can be arranged over a large area because of the structural simplicity they have. Studies have been made to exploit this advantage for various applications. Applications of surface conduction electron-emitting devices include electrically charged beam sources and display apparatus.
Applications of surface conduction electron-emitting devices arranged in numbers include electron sources realized by arranging surface conduction electron-emitting devices in parallel rows and connecting them through the opposite ends of each of the devices by means of wires to form a matrix of devices (see, inter alia, Japanese Patent Application Laid-Open No. 1-031332 filed by the applicant of the present patent application). While flat panel type display apparatus utilizing liquid crystal have been replacing CRTs in the field of image-forming apparatus including display apparatus, they have a drawback that they are not of the emission type and hence required to be provided with a back light. Therefore, there has been a strong demand for emission type display apparatus. Emission type display apparatus capable of displaying high quality images include image-forming apparatus having a large display screen that can be realized with relative ease by combining an electron source comprising a large number of surface conduction electron-emitting devices and fluorescent bodies adapted to emit visible light by electrons emitted from the devices (see, inter alia, U.S. Pat. No. 5,066,883 issued to the applicant of the present patent application).
An electron beam appratus that can be used for image-forming apparatus as described above typically comprises an envelope for maintaining vacuum within the apparatus, an electron source arranged in the envelope, targets to be irradiated with respective electron beams emitted from the electron source and an acceleration electrode for accelerating the electron beams directed to the respective targets. In addition to the above components, it may additionally comprise spacers for supporting the envelope from the inside against the atmospheric pressure.
The arrangement of such spacers within the envelope is indispensable for an image-forming apparatus of the above described type particularly when a large display screen is used and/or when the apparatus has to be made very thin.
When spacers are used within the envelope, there arise problems including (1) that electric discharges occur when electron beams are accelerated by a high voltage and (2) that electron beams are deviated from the intended respective routes to miss the respective targets (a phenomenon referred to as "beam deviation" hereinafter). The latter problem can result in a displaced and/or deformed light emitting spot produced on the target of each fluorescent body of the image-forming apparatus that can significantly degrade the quality of the display image. Particularly, when the image-forming member of the image-forming apparatus comprises fluorescent bodies for red, green and blue for displaying color images, the problem (2) as identified above can entail a deteriorated brightness and a phenomenon of color breakup. These problems are particularly remarkable at locations close to the spacers arranged between the electron source and the image-forming member presumably because electron beams and charged particles generated within the envelope under the effect of emitted electron beams can collide, at least partly, with the surfaces of the spacers to produce secondary electrons which by turn electrically charge the surfaces of the spacers and disturb the electric fields on and near the spacers so that consequently the electron beams in the envelope are deviated from there intended respective routes.
In an attempt to bypass this problem, there have been proposed a number of techniques for removing the electric charge of the spacers by using an electrically conductive material for the spacers.
For example, Japanese Patent Application Laid-Open No. 57-118355 describes a method of coating the surfaces of the plate-shaped spacers having holes at locations respectively corresponding to the thermionic cathodes with tin oxide in order to remove electrons adhering to the wall surfaces of the holes in an image-forming apparatus comprising thermionic devices. It also described that the electric conductivity of the spacers is such that an electric current between 10 pA and 0.001 .mu.A flows when a voltage of 10 V is applied between the electrodes arranged oppositely with the spacers interposed therebetween.
PCT/US94/00602 described the use of electrically conductive spacers having a secondary electron emitting efficiency close to 1 in order to minimize fluctuations in the potential of the spacers. Such electrically conductive spacers have a sheet resistance of 10.sup.9 to 10.sup.14 .OMEGA./.quadrature. and a film thickness of 0.05 to 20 .mu.m and are made of chromium oxide, copper oxide, carbon or the like. The inventor presumes that fluctuations in the potential of the spacers are caused by emitted secondary electrons and defines the potential deviation .DELTA.V at a position separated from distance x on a spacer held in contact with the substrate of the image-forming apparatus by formula (1) below; EQU .DELTA.V=.rho..sub.s .multidot.[x.multidot.(x-d)/2].multidot.j.multidot.(1-.delta.)(1),
where d is the height of the spacer (distance between the device substrate and the acceleration electrode), .rho..sub.s is the surface resistance of the spacer, j is the current density colliding with the spacer surface and .delta. is the secondary electron emission efficiency of the surface of the spacer.
Japanese Patent Application Laid-Open No. 57-118355 as cited above uses plate-shaped spacers having holes and defines the electric conductivity of the spacers in terms of the electric current (between 10 .mu.A and 0.001 .mu.A) that flows through the spacer when a voltage is applied between the oppositely disposed electrodes with the spacers interposed therebetween. Thus, the current flowing region of each of the spacers varies depending on its profile and the above definition cannot be applied to spacers having a profile other than the one described there.
As for the technique of PCT/US94/00602, if the presumption that secondary electrons are mainly responsible to the electric charge of the spacers is correct, the potential of the spacer surface varies from the ground to the acceleration voltage that is typically several kV depending on the position on the surface and hence it is practically impossible to select a material and a set of conditions with which the secondary electron emission efficiency is substantially equal to 1 over such a wide energy range. In other words, a potential deviation inevitably appears at least on part of the spacer surface. Additionally, while the electric charge of the spacers can be reduced by using a highly conductive material for the spacers, the use of such a material is not practically feasible in terms of power consumption rate of the image-forming apparatus.
As a result, positional deviations and electric discharges occur when the electron-emitting devices are on for driving such an image-forming apparatus.